In a class of conventional mass spectrometry techniques, ions having mass-to-charge ratios within a selected range (or set of ranges) are isolated in an ion trap, and the trapped ions are then excited for detection. In conventional variations on such techniques, ions trapped during a first (mass storage) step are allowed or induced to react (or dissociate) to produce other ions, and the other ions are excited for detection during a second (mass analysis) step.
For example, U.S. Pat. No. 4,736,101, issued Apr. 5, 1988, to Syka, et al., discloses a mass spectrometry method in which ions (having a mass-to-charge ratio within a predetermined range) are trapped within a three-dimensional quadrupole trapping field. The trapping field is then scanned to eject unwanted parent ions (ions other than parent ions having a desired mass-to-charge ratio) consecutively from the trap. The trapping field is then changed again to become capable of storing daughter ions of interest. The trapped parent ions are then induced to dissociate to produce daughter ions, and the daughter ions are ejected consecutively (sequentially by m/z) from the trap for detection.
It is often useful to apply broadband voltage signals to an ion trap to eject unwanted ions from the trap during performance of any (or all) of the ion storage, ion reaction or dissociation, and ion analysis steps of a mass spectrometry operation.
For example, U.S. Pat. No. 5,134,826, issued on Jul. 28, 1992 (based on U.S. Ser. No. 662,217, filed Feb. 28, 1991), describes a mass spectrometry method in which a filtered noise signal (a broadband voltage signal which has been filtered in a notch-filter) is applied to electrodes of an ion trap. The filtered noise signal can be applied during the mass storage step to resonantly eject all ions except selected parent ions out of the region of the trapping field. After application of the filtered noise signal, the only ions remaining (in significant concentrations) in the trap are parent ions having mass-to-charge ratios whose corresponding resonant frequencies fall within a notch region of the frequency-amplitude spectrum of the filtered noise signal.
U.S. Pat. No. 4,761,545, issued Aug. 2, 1988, to Marshall, et al., also discloses application of a broadband signal to an ion trap during performance of a mass spectrometry operation. Marshall et al. teach (at, for example, column 14, lines 12-14) application of a broadband signal having a notched excitation profile to an ion trap during a mass storage step (preliminary to excitation of ions of interest for detection). Marshall et al. teach the following multi-step process for generating the notched broadband signals disclosed therein:
1. selection of a mass domain excitation profile (which requires prior knowledge of the masses of both "desired" ions to be retained in a trap during application of each notched broadband excitation signal, and "undesired" ions to be ejected from the trap during application of each notched broadband excitation signal); PA1 2. conversion of the mass domain excitation profile into a frequency domain excitation spectrum; PA1 3. optional "phase encoding" of the components of the frequency domain excitation spectrum to reduce the dynamic range of the notched broadband excitation signal produced during the fourth step); PA1 4. application of an inverse-Fourier transform to convert the frequency domain excitation spectrum to a notched broadband time domain excitation signal; and PA1 5. optional weighting or shifting of the time domain excitation signal (as described in Marshall's column 3, lines 50-53). PA1 (a) generating a first sinusoidal (or other periodic) frequency component signal having a first frequency, a first amplitude, and a known phase angle relative to the start of the broadband waveform segment being constructed; PA1 (b) generating a trial signal by adding the first frequency component signal to a previously determined optimal frequency component set, and generating a dynamic range signal indicative of the trial signal's dynamic range; PA1 (c) incrementally changing the phase angle (not the frequency) of the frequency component added to the optimal frequency component set during step (b) (the "trial" frequency component) to generate a new trial frequency component; PA1 (d) subtracting the trial frequency component from the trial signal generated in step (b), and replacing said trial frequency component by the new trial frequency component to generate a new trial signal, and generating a new dynamic range signal indicative of the new trial signal's dynamic range (in preferred embodiments of the invention, the value of the new trial signal's dynamic range is recorded); PA1 (e) repeating steps (c) and (d) for each of M different phase angles which span a desired range, to identify one of the trial signal and the new trial signals which has minimum dynamic range as an optimal trial signal, and identifying the frequency components of the optimal trial signal as an expanded optimal frequency component set (in preferred embodiments of the invention, the frequency, amplitude, and phase of the frequency components of the optimal trial signal are recorded); and PA1 (f) repeating steps (a)-(e) for an additional sinusoidal (or other periodic) frequency component having a frequency different than that of any frequency component generated during a previous repetition of step (a).
Use and generation of time domain excitation signals as taught by Marshall is subject to several serious disadvantages, including the following. First, Marshall's technique for generating a notched broadband signal requires prior knowledge of the masses of both desired ions to be retained in the trap during application of the signal and undesired ions to be ejected from the trap during application of the signal. Marshall's technique for generating a notched broadband signal also requires construction of a complete mass domain excitation profile waveform in order to generate a time domain excitation signal for each mass spectrometry experiment.
Also, undesired missing frequency components ("holes") can result during conversion of Marshall's mass domain excitation profile into a frequency domain excitation spectrum. The risk of such undesired holes is enhanced due to the inverse relationship between mass and frequency (so that if Marshall's mass domain excitation profile has closely spaced undesired mass components corresponding to undesired ions having high "q" values, the corresponding frequency components of the frequency domain excitation spectrum generated from the mass domain excitation profile will be widely separated). Undesired holes in a notched broadband excitation signal resulting from Marshall's technique can leave unwanted ions in the trap following application of Marshall's notched broadband excitation signal to the trap.
Conventional techniques for reducing dynamic range of a broadband signal have selected a functional relationship between phase and frequency, and assigned the phase of each frequency component of the broadband signal in accordance with the selected functional relationship. For example, the phase encoding technique disclosed in Marshall (at column 9) requires selection of a nonlinear functional relation between phase and frequency, and assignment of phases of the frequency components in accordance with this functional relation. Other conventional techniques for reducing a broadband signal's dynamic range have randomly selected the phases of the frequency components of the broadband signal in an effort to randomly select a set of phases which results in reduced dynamic range. Neither of these conventional methods for generating a broadband signal with reduced dynamic range is mathematically precise, and neither allows for true optimization (i.e., dynamic range minimization) of the resulting broadband signal.
It would be desirable to generate notched broadband signals, each having low (and preferably minimized) dynamic range and frequency-amplitude spectrum specifically designed for a particular mass spectrometry operation, in a manner enabling rapid generation (for example, real time) of a sequence of such signals (for use in a sequence of different mass spectrometry operations) without significantly impeding the performance of such sequence of mass spectrometry operations. It would also be desirable to generate such notched broadband signals without a need for prior knowledge of undesired ions to be ejected during application of the notched broadband signals. It would also be desirable to generate many different notched broadband signals for many different mass spectrometry experiments, by performing rapid processing operations (for example, in real time) on a single broadband signal (having optimized dynamic range).